A user interface and method is embodied on a computer readable medium and executable on a computer. The user interface is a labeler which labels only foreground pixels of an image stored in a computing environment. The labeler operates in a Region mode/state and Brush mode/state, and includes a Tentative mode that permits an assigned label to be changed after pixels have been selected. Groups of pixels may be selected for labeling at once by a point-and-click command, and a pixel may belong to one or more groups of pixels which are stored in memory as image layers. The groups are formed dynamically by user selection actions, and/or through automatic recognition algorithms. Pixels already labeled with certain labels may be locked to not be altered by additional labeling operations. Unassigned pixels may be highlighted to increase the ease at which they are identified in an image. Comparisons between labeled images are undertaken to indicate differences between different groundtruth labeling.